If your subjects and dividers disappear, these steps should let you restore your subjects. We don't have a way to restore dividers currently, but we're working to prevent this issue.
- In Notability on an iPad or iPhone, go to the screen that lists your notes.
- Press and hold on a note's name.
- Tap the "Duplicate" button that appears.
- Delete the duplicate copy of the note.
- Restore the note from Notability's trash.
- This will restore the note's subject and put all of the relevant notes back in the subject. As a result, so you should only have to follow these steps once per subject.